Gravity meter



Dec. 21; 1943. .D. H. CLEWELL- 2,3

- GRAVITY METER Filed July 12, 1940 3 Sheets-Sheet l l I jwuc/Moo I g l I q fiqy/m/Zflewell &

Dec. 21,1943.

. D. H. CLEWELL GRAVITY METER Filed July 12, 1940 3 Sheets-Shee't 3 MMIMAMAAA I, MIIIHH tours r thesubsurface strata. Such instruments Patented Dec. 21, 1943 Y =-g tic means. must-rhea free, of instruments of the type used in locating abnor andihYSbel'eSiSQflECtSRzfilldg malities such as discontinuities that are occayond thepoint where its desirable elastic prop l; sioned by faults, anticlines and other structures erties' deteriorate ismlsoqequired gthat such as salt domes More particularly, this inelastic means must support the beam in such a ventionrelate'sf apparatus for ni asurin v manner that very small changes in gravity will directly variations ingravitational "force' from produce deflections cof s-theebean'ixof such magni-r; point'to pointover the earthsfsurfaceq tudefthatthey arereadily observableby means-:11 'Ithas long been known-to geologists and'others oira conventional optical systemgsuch aswa: mi..- skilledin the art'oi"geophysioalprbspeotingthat 10 croscope, .or :other-cmeans forzrdeterminingether, variationsin gravitational 'iorcefrom ointto D0siti0n er thebeam:;;Additional1y; it'is, desilableso point over the-"earth's surface are directly related" to make I the. beam'as stablesas possible to insurers: to'the disposition of the -substrata witli respect; reliable pertormancesoft' thei instrument inefieldfii to 'the'earths surface: ther fore; by measuri g workiir. 12:1 :1; the variations in gravitationaliorce from aint- 1 In a United .iStates; Patents No to point over an area otthe earth's surface and Trumani and No. :2,108,421 to ThysseneBorne-m plotting these dataon mapsinthe form of con-' misza, forms otmgravity: injeters i 'are,.disclosed:z:. tours, a map is formedwhich simulates the con-' wherein itwis. intended; :that zpthe :advantagesa ctr high gravity sensitivity, j be tobt'ainecl' gin 11a ;=prac 1 0 ticalmand usefulsniannerziiThesezrin'vtentorsyzin e theiri disclosures faileto, considenthe' :question rot-1:: stability to adjustment and low level sensitivity in connection with thdli'se'df a single spring and, .as a result, do not dislosetthe un'c'ierlying prin'i 5 ciples of the instant in ven on. 5 Itis a primary object of this invention to provide a novel apparatus whereby the variatioin gravitational ierce frofii -point to pointrou hout-aw f QQQ HIbS? tem ave en men -w; belvqll e f l lifi d h y, being i'used" in], the industry s ee' ssruu in "ct ducting thereconnaissance.lgeonhysical; su l W Experimental work has shown thatthe pivoted horizontal ,bearn} gravity, meter; .iwheni ,prope' designjedjarid cons'trlllq .J1?- ml n y l charaote istios that make'it va1uable"rapidsurvey tr t e al teamer af il' t 'ih zentel e m avit mefienr e s I' -e112 r he eet w er he en er, ei' il r 2 an ete' responsive means is free to rotatje through a sin angular a v n, a v t ca plane iab i 1 lni otal' axis inoluded in' sub'sta itially'lth ifihi A sponsive means is elastically supporteg in a si n ple and compact manner by a single pretens'ioned coil spring; 45-" Other objects of this "vention reside in the eiproviaion of various eet damping of the gravity responsive beam, and rigid clamping of 'ihe beanr qu iir g transportation, to observe the deflection f the beam and F various other means necessa' ,gtO make the gravi ity rnetenai ere atiya tm gie it as EiILPQfiYia The horizontal piyrotegi ,p e gr vity meten defrt'f, t e f'giliofw ii gfihetiiea ,desenpae when proDerIy ccYiiStiuctd,"inherently circum whe'ireonsiilerd'" with t p nt names e tit ement ret era-tena ts! a Figure 1 w diagram developing the theory 0 and from abnormally higher sensitivi the horizontfil pi v ot egl.beaifigiavitiiifiieter;

' tain st ay forces t a tgto he ij f a i Figure 215% diagram illustfating tli ifiinimum The designer of a fi W5 useful horizontal h i iifiiilfiiti iibe t 1 1% ta te-a itate glimmer- 5351i m type of a ty m t r must, th e. pr Figure 3 is a side elevation of a referred ar videeclastidimea ie etor ,sunpnminenthc rangement of th lssgllfigL'iemegj ofme gravedixilibriumsaiitlaixtheriorceiotrs reiiiiygmmsielcs rmfl ity meter in operative hregatignlghip;

.eii siciieefish meted sidisiiaene swat e: a

e ihaiviriiis ifi Figure 4 is a side elevation of another arrangement of a horizontal pivoted beam gravity meter; and

Figure 5 is a perspective view'of a novel pivoting means that is free of friction.

The general theory of the horizontal pivoted .beam can be briefly outlined as follows: In Figure 1, a truly level surface is represented by the line PQ, while the line MN making an angle 5 with PQ is a reference line fixed to the framework of the instrument relative to which the gravity responsive beam is rotatable. The axis of rotation is horizontal and intersects the horizontal line PQ at O. The center of gravity of the beam is at C, a distance r from O. The angle between 00 and PQ is w and the angle between 0C and MN The gravity moment of the beam is then mgr cos a which is balanced in some way by an elastic torque r applied to the beam. The elastic torque is a function of 0, rather than in, since the elastic means used to generate the torque must obviously be anchored to the framework of-the instrument and 6 is the angle denoting the orientation of the beam relative to the framework.

For a general discussion no further specification of the details of the origin and application of the elastic torque is necessary. The instant invention, as will be described later, will be concerned with novel arrangements of the elastic means generating the torque 1'.

For equilibrium of the pivoted beam to exist the torque must equal the gravity moment, or,

I. -r=mgr 008 a: From Figure 1 we note that thus Ia. -r=mgr cos (0+5) This equation is a relation that describes theangular position- (0) of the beam as a function of gravity (9). Any means used that is indicative of the position of the beam will naturally be attached to the framework of the instrument so that 0 will be the immediately observable parameter.

Conditions to'impose on the parameters of the' beam to insure ,a detectable change in 0 will result from a very small change in gravity, are essential for a useful instrument.

The gravity sensitivity S can. be defined as the ratio of the beam deflection (dB) to the fractional change in gravity producing the deflection, or,

do I g II. iS 1. 01' d0 S 9 To evaluate 8, Equation I is diflerentiated considering 0 and a as variable and 18,8 a function of 0. v

(11' d0 mgr cos a: d0 E 6=-g mqr S111 ai -g Rearranging the terms of the above equation:

do: mgrcos w 9 1' d-mgr sin w v that in any'usable instrument S is large, but not I infinite. Thisprocess of securing high ensitivity which when compared to Equation II shows that In mgr cos w T -lmgr sin to is of the order of 10 To obtain this condition,

S is usually of the order of 20 to 200 if high resolving power optical means are used to detect the deflections of the beam. These high values of S are readily obtained by proper selection of the parameters in Equation III. In fact, it is possible to cause the beam to be infinitely sensitive to gravity by choosing the parameters of Equation III in such a mannerthat the denominator becomes zero; i. e.,

Infinite sensitivity is of course impractical so by approaching the condition described by Equation IV has been described in various ways in the prior art; labilizing, astatization and inducing a long period of oscillation are common terms that have been used. I

By a further consideration of Equation I it is possible to learn how the parameters of the beam such as -r, w, 1', etc., may be chosen to minimize level sensitivity. In Figure 1 it is evident that the angle p represents'the orientation of the instrument with the true horizontal surface. Errors in levelling will appear as variations in fi. Level errors in the perpendicular direction will appear as variations in the angle which the axis of rotation of the beam makes with the horizontal. This angle will be denoted as 5. In the development of all the previous equations it has been assumed that the axis of rotation was horizontal and 5' was equal to zero. To discuss level sensitivity it will be necessary to first assume the axis of rotation is slightly out of level by an amount p and then to determine what effect variations in p. as well as in ,9, will have on the deflection of the beam.

When ,9 is'different from zero, Equation Ia must be modified to allow for a decrease in the gravity moment of the beam when its plane of rotation is not exactly'vertical. Equation Ia becomes Ib. r=mgr cos (0+6) cos a" For minimum level sensitivity it is then 'required that 1 i B approach a minimum value, zero if possible, thereand by indicating that level errors ds and d5 will cause a negligible beam deflection do.

, 2,887,152 Dlflerentiatlng Equation Ib, considering 0 as a "cdndltlons of Equation IV and the beam is un-- mm pretax-ed: 11931128215 5 msit mi wh re'w e d meter of this :1? ithe tgeam; 55%

rating ran e afl'husilminlmum 1e,v,e sensit .M e e. U operntlnsimhewbeamiimsucn; aqp tlone hatvw essentially zero degrees andpmmaingamlpgfthe axiswt mtatiominmhonzonta yplane; thaw isess n l1 zerzohwhezeuppnzb t in he, d-le nthsqrey el au h so the 8 1 3161 mmmegens tpiatedt .suc

"new m Mia operation.

structure I0. edge I5 in a substantially horizontal direction to free of undue level sensitivity; that is, a null mean's must be provided for maintainin to equal to zero. The necessary adjustment of the nulling figures, for purpose of explanation, illustrate detailed constructions which utilize the theory in In referring to Figure 3 of the drawings, there is shown pivotally mounted for rotation in a vertical plane a mass structure I within a housing I I. The pivot for the mass structure I0 is formed in part by a block I2 of hard material such as agate, secured to a support l3 which in turn is carried by a side I4 of the gravity meter housing II. Block l2 has itsouter surface polished to receive 'a knife-edge I5 formed at the junction of arms. I6 and ll of the mass Arm l'textends from the knifethe major portion I8 of the mass structure Ill. The center of gravity C of the mass structure and the pivotal axis 0 are arranged to lie in the same horizontal plane. Arm I6 in this particularform of the instrument extends upwardly from the knife-edge I5 at an acute angle to the arm I1 and is provided at its outer end with a clamp I9 adapted to receive one end of a pretensioned spring 20. Pretensioned spring 20 serves to supply the elastic forces necessaryto maintain the mass structure in equilibrium. The opposite end 20 of pretensioned spring 20 is secured to an anchor 2| that is slidably mounted within an anchor slide 22. Anchor slide 22 is also slidably mounted on a support 23 carried by the wall I4 of the gravity meter housing. Elements 2| and 22 permitadjustment of the end 24 of the pretensioned spring 20 to any point within a limited vertical plane so as to obtain proper adjustment of the spring. Adjustment of the screw 25 serves primarily to adjust the gravity sensitivity of the mass structure I0 and adjustment of the screw 26 serves primarily tobring the mass structure into the normal operating range, and in some I instances may be used to null the instrument.

Means indicative of the position of the beam comprise microscope 21, .scale 28, and pointer 29. Pointer 29 is attached to the brace arm 30 of the mass structure and cooperates with the scale 28, which is adjustably mountedto the side I 4 of the housing, so that the relative position of the mass structure can be observed by the microscope 21. Scale 28 is socalibrated and adjusted relative to the indicator 29 carried by arm 30 of the mass structure that when the center of gravity 0 of the mass structure and the pivot 0 formed by knife-edge I5 and block l2 are in the same horizontal plane, indicator 29 will be at the mid-point or zero point of the scale 28. Thus the normal operating range of the mass consists of small angular deflections above or below the zero point. In order to maintain the working elements of the instrument as insensitive to level as possible, a nulling means must be provided for zeroing the pointer 29 after the mass structure has been displaced by a change in gravity. To

this end there is provided a spiral spring 3|, the

. outer'end of which is secured to the pretensioned coil spring 20 at a point near the end 24 thereof.

The inner end of the spiral spring is secured to 1 a shaft 32 journaled for rotation in bearings, not shown, carried by side H of the gravity meter 2,337,152 ating range of the beam within limits that are housing. Rotation of the shaft 32 will cause the spring 3| to move the body portion of the pretensioned spring 20 in a. transverse direction to vary the eifective lever arm through which it acts on the mass structure I0. The amount of effort necessary to displace the body of the spring an amount sufilcient to return indicator 29, carried by the mass structure I0, to the zero position on the scale 28 will be a measure of the gravity change which produced the original displacement of the mass structure. There is provided, fixed to the shaft 32 and adapted for rotation ,therewith, an indicator 33 which cooperates with scale 34. Scale 34 can be calibrated directly in terms of gravitational force. Although this spe-, cific type of null system is shown, it is obvious to those skilled in the art that various other systems may be used.

Because of the extreme delicateness of the elements forming this apparatus, it is necessary that some means he provided for rigidly clamping the mass structure when the instrument is being transported from one location to another. For purpose of illustration there is shown a block 35 secured to the bottom side 36 of. the gravity meter housing II. Block 35 is so formed that it presents a face 39 having a plurality of fingers 38 extending therefrom to a face 3Iformed on the main portion I8 of the mass. A second face 40 is formed parallel to face 31 but on the opposite side of the main portion I8 of the mass against which some means; such as a screw II, can exert a force to rigidly clamp the mass against the fingers 38 carried by the block 35. Screw 4| can threadedly engage a support 42 carried by the side I4 of the gravity meter housing I,

ceive the spring 20 when a bar-shaped element I03 is pressed against the spring through the agency of the screw I04. Screw I04 is threaded into the support I05 which is also mounted to the side I4 of the gravity meter housing. Preferv ably the bar- I03 carries a layer of soft material such as leather I06 on its face adjacent the spring. By operation of the screw I 04 the bar I03 can be used to gently press the spring into the trough |0| whereupon the spring is prevented from injury that might occur from shocks given the instrument during transportation. It is to be understood that although screws have been shown for both clamping mechanisms, they are merely for purpose of illustration and in actual construction of the instrument can be replaced by other means more conventionally adapted to the clamping operation.

In order that the mass structure may be quickly brought to rest after it has been unclamped, damping means 43 are provided. Damping means 43 consist of a cup-shaped member 44 carried by the arm 30 of the mass structure and an element 45 that is rigidly secured to the side I ofthe gravity meter housing. Element 45 is provided with an annular groove that is adapted to receive the open end of the cup-shaped member,there being sufficient clearance provided that the cup-shaped member 44 will not contact the element 45, but will merely form air passageways t me n trie . e em ie ee jstmscessity must be below the hortggntal plane pagworethesapirgmgnnmliahemte fimfiemmenifls 1n: through the pivotal axis and ce ter 0! gravity the Wt C1 of the mass structureywdmd flint arm 50.51a mwmbgmsmtmuwd provided with a clamp by means ofjwhlch end 63 gmmwnmtmtesnv a bqms smem el mm the fi geggtggu Qi ii flmqgglfibe secured to mmxmwmfi emmmb *xslusisveeq and nectar the arm 5|. e 05 1? te end o! spring 3| as'xebeultml's ands-Q11; mm QMMiDmln H .nn; --ztheasahiegnanneh:amdteactihqdli? qlmwtinwawith 3min Q1: Q1 6% ad. has "=1 germ: ed; 2M5 ,ne gas ieifip etm wawemsat-w e;

quire that some means he prpvlded whereby the the mass structureiie on one sldeot the pivotaxls memeiitiw l me tet teatze aes eeetqe mess atzucture can be rlaidly damped when the ll therebymakinz 1t poeslble to: the center ot'grav.

In conducting a geophysical survey by the gravimetric method, a surveyor first lays out the necessary number of suitably distributed stations over the area. One of these stations is then selected as the base station and the force of gravity measured at all the other stations is compared with the gravity at this station. In operation of the instrument in the field the gravity meter housing, suitably enclosed in a constant temperature oven, is set up on a tripod at the base station where the gravity survey is to be started and levelled by means of the level vials 46 and 4! The mass structure is then unclamped by backing off the screw II to permit the mass to swing free of the fingers 38. In order that the mass struc-- ture may be in its normal operating range for the particular area to be surveyed, screw 26 is adjusted until the pointer 29 is within the reading range of the scale 28. For the remainder of the survey no further adjustment of screw 26 is made. To bring the mass structure to the exact zero position, the nulling system is used wherein the shaft 32 is rotated until spiral spring 3| exerts suficient effort on main spring 20 to zero the mass structure. After the mass structur has been zeroed the position of the pointer 33 on the scale 34 is the gravity reading at this first station,

commonly called the base station. To compare the force of gravity at other stations within the area with the force of gravity at this base station the instrument is transported successively to the other stations, the mass structure and pretensioned spring being clamped during the transportation. At each station the mass structure and spring are released from their clamps and scribed above possess the desirable characteristics which the underlying theory indicates as desirable, reference is again made to Figure 1 and Figure 3. Letthe reference line MN in Figure 1 which has been described as a-line fixed to the instrument housing, be represented in Figure 3 by the line a joining the pivot axis, 0 with the end 24 of the pretensioned iipring 20. Let the angle between line-a and the plan determined.

by the center of gravity C of the mass structure 6 and its axis 0 be denoted as 0.- Also let the angle between a line joining the axis 0 with the end of the pretensioned spring secured by the clamp l9 and again the plane determined by the center 01' gravity C and the axis 0 be a. It is obvious that 6 is a constant depending upon the particular dimensions that were used in building the mass structure. Let the force F exertedby the 'pretensloned spring 20 be expressed by the equation where his the length of the spring for which it exerts the forceF and L0 is the length of the spring forwhich the spring exerts no force; k

commonly known as the spring constant, is, by-

definition, the force necessary to extend the spring a unit length. L0 is commonly called the initial length of the spring. In manufacturing the spring Lo may .be controlled in magnitude relative to the working length L by the amount of pretension incorporated into the spring. The torque generated by the pretensioned coil spring 20 will now be the forc F multiplied by the effecv tive lever arm on which the spring acts. From th geometry of the arrangement in Figure 3 it is obvious that the torque 1- is where a is the distance from; th pivot axis 0 to the end 24 of spring 20; b is the distance from the axis 0 to the other end of the spring 20 and the other parameters are as previously described. This same formula is true of the arrangement shown in Figure 4 if the angles 0 and 5 are measured as shown on Figure 4 and if a is now taken as the distance from the pivot axis 01 to the end 63' of the spring 6! and b is the distance from the axis 01 to the end 63 of spring 61.

A consideration of the above equation shows that the angles 0 and 6 occur always in combination as (0-45). Therefore, the torque function 1' is independent of the particular values of 0 and 6 so long as (0-6) is unchanged. For example, in Figure 3, the angle 9 is illustrated as being approximately 135 and 6 is illustrated as about 45, in which case (0- 6) 15' 90. In Figure 4, 9 is shown as approximately 315 and 6 as 225, so that again (0-6) is about 90". It isthus apparent that the torque function is the same for both, arrangements and in fact woul be reproducedin form identical with Equation X for any disposition of the sprlng'relative to the mass structure; that is 0 and 6 could range through all values between 0 and 360? and as long as (0-6) is substantially equal to 90 the arrangement would be equivalent to the two examples illustrated in Figures 3 and 4. g If in Equation X 0-6) is approximately 90, and the product lcab isapproximately equal to mgr and ii still further the pretension in the spring is suificient to render Lo small compared to L, where L= a+b'-'2ab cos (0-6) then it is true that the torque function 1- will simultaneously satisfy the three necessary conditions which the underlying theory requires in order that the instrument will be a practical and a useful gravity meter for field work, because making thes substitutions in Equation. X will,

reduce Equation X to the same form as Equation VIII. The torque function 1' generated by the particular-arrangements described will be of such form that 1.- -r mgr I 5 mgr dr 3. mgr (approximately) the illustrated arrangements shown in Figures 3 and 4 two types of pivot structures were used. In Flgure3 a knife-edge pivot was used,

= while in Figure 4 a pivot structure of light flexural members were used to serve as a pivot. It is of course understood that either type of pivot but there will be no fundaniental change in th izt iltawsi fi th e qt he tru n ri parallel to the side of the gravityin eter housing to which it is secured, said scuring means and the means whereby the elastic meansis clamped to the assis xuntu s be nsi i-disb relative to the pivotal axis of the' mass""structure that adines-idrawnathizough the ipiyotal axis to opposite ends oiritheaelasticgmeansswill; form zwithlzgeach t aowgothemapproximately:aailflhansle mhereby advan- M38885: of s highistahilityangimfrhig ares/1t! sensitivity :anei:attained.r new: h e x mwaifiassravityzmeterh QHQDI-lBl-HE v upward component gqg sb a housing, a mass structure id sposedizw thmithe filvsupporty yx the housingmneansrmr piyotall mounting; h mfl brstlucturegiffiwmtationimtia 89. 3149 n ginesisaid s nrassi.astmcturjeimay-ins:w its centerco ravlt Qlfl emivotaimxisiin substantiallnit eaisamazhoriznntai emlanetscthatithermasssstructure'iieflccti nsmlll idii'be =substantiailyi-independentint nthe orien aflw ofiithealiousinzsimia gravitational fieldns cans eiionrelasticallyrbalancinsa:thezisrayi .mcme on! asthema'ssstructure,isaidsmeans compnlsinx tensioned helihalsprinxfli' sclainp c rried-mysth mass structure adaptedto receive 'one end of said spring, an adjustable @a'hchor secured to one side of the housing adapte to receive the opposite gravity of the mass structure in such a manner A W ttfs vantnvwerkizig eieinsntsmi themvi v m t *fi e l l'i is mehnamhsi 'ilsiinsi crii' ism zesurnmseeuim e iesasen w mat passes throug-h' the end oi the sprink andiis r 1 W, J i x a P a it w a nensiss itiet t ness mi f ifi natsy hiqe imnm a 1m? the mustmcturjedm w mmm-edflnsmon E3121??? ti'iiscma maximiz 15m being measurable gravity 2 mgr p 4. A gravity metevcom grising in combination aw "a tann n e r; 1 m a l wing g m gss structure disposed v ithin the fiic *6 mimml in iio uslhtfimeaiis' ib? pivttaihr inountinfi twm mgr pp y structure for rotation in a vertical plane, a prewhere mm is the gravity moment or the mass tensioned helical spring secured to the mass structure and 0 is the angle of rotation ot'the structure for balancing th gravity moment 01 mass structure, and means for varying the eflecthe mass structure, said mass structure being so tive lever arm through whi h th balancing disposed relative to its pivotal axis in balanced means acts upon the mass structure to displace ll condition that its center at gravity lies in subg the pretensioned helical spring will form an angle stantially the same horizontal plane as the pivotal axis, said pretensioned helical spring being disposed entirely below a substantially horizontal plane passing through the pivotal axis and the center of gravity of the mass structure, means in addition to the mass structure for securing the other end of the pretensioned helical sprin in such a manner that lines drawnfromthe pivotal f structure and is the angular disposition of the axis of the mass structure through the ends of of substantially 90 when the gravity meter has been adjusted to a high gravity sensitivity.

5. A gravity measuring device comprising a mass structure, means for pivotally mounting the mass structure for rotation in a vertical plane about an axis in substantially the same horizontal plane as the center of gravity of the mass structure, a pretensioned coil spring disposed entirely below a substantially horizontal plane passing through the center-of gravity of the mass structure and its pivotal axis for balancing the gravity moment of the mass structure, which generates a torque -iof such form that (1 1 Z165: mgr (approximatel y) ;where' mgr is the gravity moment of the mass 1 structure and 0 is the angle of rotationoi the mass structure, and means for varying the efiective lever arm through which the pretensioned coil spring acts uponthe mass structure to displace the mass structure an amount suiflcient to null the-instrument.

housing,- a mass structure pivotally mounted as 6. A gravity measuring device comprising a therein for rotation in a vertical plane about a horizontal axis, elastic means attached to the mass structure and to the housing for balancingthe gravity moment of the mass structure and for supporting it so that its center 'of gravity is in substantially the same horizontal plane as the horizontal axisof rotation, said elastic means generating a torque T such that I u g d0 7 is small compared with the gravity moment of the beam, 0 being the angular disposition of the mass structure relative to the housIng and a nulling means capable of returning the mass structure to the position in which its center of gravity is inthe same horizontal plane as the axis of rotation, whereby the device may be mainmass structure relative to its housing, and a nulling means capable of returning the mass structure to such a position that its centenof gravity is in the same horizontal plane as its axis of rotation in order that the device always 'be in a condition of minimum level sensitivity.

8. In a gravity measuring device comprising a housing, a mass structure therein mounted upon a flexible pivot for rotation in the vertical plane about a horizontal axis, elastic means attached at one end to the mass structure for balancing the gravity moment of said mass structureso that the center of gravity thereof is substantially in the same horizontal plane as the horizontal ,axis of rotation, means for attaching the'other end of said elastic means to said housing, said elastic means generating a torque T. such that a is 110 is small compared with the gravitymoment of the beam, 0 being the angular disposition of the mass structure relative to the housing, in combination with a nulling means capable of returning the mass structure to the plane of the axis as the gravity forces acting on the mass vary, and means for adjusting the position of the means attaching the elastic means to the hous-. ing whereby the center of gravity of the mass structure may be initially adjusted to lie in the same horizontal plane as the axis of rotation about the flexible pivot and returned to said 'plane as the gravity force varies to maintain the adapted to receive one end of the said spring,

means secured to one side of the housing adapted to receive the opposite end of said spring, means for indicating displacement of the mass structure effected by changes in the gravitational force,

means for nulling-the instrument, and means for indicating the amount of eflort required to return the mass structure to a predetermined "position, the force required to return the mass 'tained in a condition of minimum level sensi tivity.

. 7. A gravity measuring device comprising a housing, amass structure,-means ifor pivotally mountins the mass structure for rotation in a vertical plane'about a horizontal axis, with the centerof gravity -01 the mass structure in the same horizontal plane as the axis, elastic means attached to the mass structure for balancing the gravity moment of the mass structure, said elastic means generating a torque T such that where mgr is the gravity moment of the mass side of the housing is adjustable, and the adlift structure to a predetermined position being measurable in terms of 'gravity, characterized by the facts that the pivotal means are flexible, the mass structure has its center at gravity and pivotal axis in substantially the same horizontal plane so that the mass structure deflections will be substantially independent of the orientation" of the. housing in the gravitational field, the means forsecuring the end of the spring to one justable securing means, spring and clamp all lying on one side of the horizontal plane passing through the pivotal axis and the center of gravity of the mass structure in such a manner that all the working elements of the gravity meter will occupy a minimum amount oi. space.

" DAYTON n. CLEWELL. 

